As apparatuses adapted to analyze the concentrations of hydrocarbon (HC) and nitrogen oxide (NOx) contained in exhaust gas discharged from, for example, an internal combustion engine, there are ones using a hydrogen flame ionization analysis method (FID) and a chemiluminescence analysis method (CLD), respectively.
When a required range for measuring the concentration of a measurement target component is wide, such as in the case of FID or CLD, to deal with such a situation, the amplification factor of an amplifier for amplifying an analog signal outputted from a detector is switched. For example, when the concentration of a measurement target component is high, a small amplification factor is used, whereas when the concentration is low, a large amplification factor is used. In addition, when measuring a measurement target component of high concentration, the linearity between the analog signal outputted from the detector and the concentration tends to be lost, and therefore it is necessary to use a high order calibration curve coefficient.
When measuring a measurement target component of high concentration, the above-described measures may be taken; however, the following problems exist.
That is, a large amount of the measurement target component (molecules) flows into the detector, and consequently, an introduction path to the detector may be contaminated to increase a background level. Also, the amplifier is required to have multiple amplification factors, causing one of adverse effects on miniaturization of a circuit board. Further, as the concentration of the measurement target component increases, the linearity between the analog signal outputted from the detector and the concentration is lost, and therefore it is necessary to ensure the linearity by employing a high order calibration curve coefficient.